The prior art relating to the molecular orientation and heat-shrinking processes of thermoplastic saturated linear polymers, such as polypropylene, polyethylene or polyethylene terephthalate ("PET"), is extensive. It is well known in the art that films or tubes of unoriented thermoplastics may be heated to their orientation temperature and "stretched" in order to "orient" the linear polymeric chains. Such orientation greatly increases the strength of the material in the direction of stretching. By simultaneously or serially stretching a film of unoriented linear polymer in two directions perpendicular to each other, a material of consistent superior properties in all directions is obtained. Such products are referred to as being biaxially oriented. Biaxially oriented thermoplastics have many desireable properties including increased tensile strength and elastic modulus.
There are two general categories of thermoplastics that are capable of orientation. The mono-1-olefins, such as polyethylene and polypropylene, are crystalline polymers. Other thermoplastics, most predominant among these being PET, are crystallizable polymers. Crystallizable polymers can be produced in an amorphous or non-crystalline solid state capable of being transformed into a crystalline form through heating to temperatures above the orientation temperature of the material. The length of time required to crystallize crystallizable polymers is dependent on the temperature and the degree of crystallinity required. Oriented then crystallized polymers have significantly enhanced thermal dimensional stability over crystalline polymers because of their heat-setting abilities.
The temperature employed in heat-setting a crystallizable polymer defines the maximum temperature to which the product may subsequently be heated without causing the polymer to relax toward its unoriented shape.
In the case of PET, the optimal orientation temperature range in which biaxial stretching occurs is between 80.degree. C. and 110.degree. C. U.S. Pat. No. 2,823,421 of Scarlett, for example, describes a method for orienting an amorphous film of PET 3.25 times its original longitudinal width at a temperature between 80.degree.-90.degree. C. The temperature of the film is then raised to between 95.degree.-110.degree. C. before it is transversely stretched. The resultant biaxially oriented film is then heat-set at a temperature in the 150.degree.-250.degree. C. range.
Although raising the temperature of oriented PET during heat-setting will "set" the form of the film, unless restrained by some means such as tenting frames, molds or air pressure, the film tends to shrink significantly during the heat-setting process. Oriented crystalline polymers will also shrink upon heating.
The heat-shrinking characteristics of oriented crystalline and crystallizable polymers is exploited by this invention to form products with unique characteristics. For either group of polymers, the shape an article is conformed to during heat-shrinking is maintained by the article after it is cooled to room temperature. A crystalline polymer will lose its shape when heated above its orientation temperature, while crystallizable polymers may be heat-set to temperatures above its orientation temperature but below its melting point.
Heat-shrink tubing for the insulation of electrical connections is well known in the prior art. Another example of a process used to capitalize on this property, the heat-shrinking of polyvinyl chloride, a crystalline polymer, for the purpose of placing a hard plastic coating on photoflash lamps, is described in U.S. Pat. No. 4,045,530 of Reiber. U.S. Pat. No. 2,784,456 of Grabenstein describes the use of bands of PET, a crystallizable polymer, to seal bottles containing beverages and foods by heat shrinking the bands over the bottle and cap juncture. Neither of these patents discloses the use of the heat-shrinking process in order to mold the shape of an article to be later used independent from the coated substrate.
Crystallizable polymers, such as PET, also may be heat-set in a non-oriented form. Raising the temperature of amorphous PET above its orientation temperature range will "set" the form of the object, producing a strong, hard but somewhat brittle material. Heat set unoriented PET is milky white and translucent and will retain its physical structure on heating to temperatures in the 200.degree. to 250.degree. C. range.
Due to the excellent strength characteristics of oriented plastics, there are a substantial number of commercially available products composed of these materials. For example, the commonly used two liter bottles of carbonated drinks are generally made of oriented PET.
Patents describing processes and apparatus for the efficient production of open ended containers made of biaxially oriented thermoplastics are numerous. See, for example, U.S. Pat. Nos. 4,711,624 of Watson; 4,381,279, 4,405,546 and 4,264,558 of Jakobsen; 4,563,325 and 3,532,786 of Coffman; and 3,412,188 and 3,439,380 of Seefluth.
The most frequently described method for forming containers utilizes a combination of injection molding and blow-forming. According to these procedures, a solution of molten thermoplastic is injection molded into a mold to form a parison or pre-form. Typically, the parison is removed from the injection mold and placed in or surrounded by a female mold. The temperature of the parison is brought into the orientation temperature range, at which time it is blow-molded into a female mold in order to biaxially orient the thermoplastic and give it its final shape.
There are several advantages in utilizing this two-step process. The portion of the parison that will be used as the neck of the container may be injection molded to contain intricate structure such as the ribbing required for a screw-on cap. This neck portion can be positioned so that its shape is retained during the blow-molding.
Once shaped, the blow-molded container may be cooled to room temperature to retain its shape. If a crystallizable polymer is used, the container may be heat-set to higher temperatures prior to cooling. If heat setting is desired, a positive pressure must be maintained in the container to prevent shrinkage during heating. For an example of this general type of apparatus and method see U.S. Pat. No. 4,108,937 of Martineu.
Another series of patents describes the plug-forming of thermoplastic sheets. Blow-forming a sheet requires that a sheet of thermoplastic material be clamped over a mold, heated to its orientation temperature and then conformed to the mold by the action of positive pressure. In plug-molding, a male form is used to assist in the conformational process. U.S. Pat. No. 4,420,454 of Kawaguchi describes a method of plug-molding followed by blow-molding to produce biaxially oriented containers.
A commonly employed method for the production of thermoplastic containers, particularly for use in the food industry, is referred to as thermoforming. Thermoforming is the formation of an article by manipulation of thermoplastic material at a temperature above its flow temperature but below its melt temperature.
In many of these systems, the process begins with a blank of thermoplastic material. The temperature of the blank is elevated to near its melt temperature and then forged into a disc-shaped preform. The peripheral edge of the preform is the incipient rim of the final container, which is rapidly cooled after forging while the bulk of the preform remains at an elevated temperature. The preform is then subjected to thermoforming and the thermoplastic attains the desired final shape. Preforms can also consist of sheets of thermoplastic, or may be produced by injection molding techniques.
The thermoforming step can be accomplished in a number of manners. In one variation, the thermoforming is accomplished by the introduction of a male plug that presses the malleable thermoplastic into a female mold. U.S. Pat. Nos. 3,499,188; 3,546,746; 3,642,415; and 3,757,718 of Johnson and 3,532,786 and 3,606,958 of Coffman, each of which is assigned to Shell Oil Company describe the plug molding variation of thermoforming.
In other variations of thermoforming, the deformation of the malleable thermoplastic is accomplished by either increasing the fluid pressure on the side of the preform opposite a female mold, or decreasing the fluid pressure in the area between the preform and the interior of the female mold, or both. It is taught that this can only be utilized when the depth of draw is minimal (e.g., when the ratio of depth of the container to the diameter of the container at the bottom is less than 1.0). See, for example, U.S. Pat. Nos. 3,739,052; 3,947,204; 3,995,763; and 4,005,967 of Ayers et al., each which is assigned to DOW Chemical Company; and 3,244,780 of Levey.
In a final variation on the thermoforming step, it is often desireable to combine plug molding and blow molding. Plug assisted blow molding is most often useful when a larger depth of draw is required or when the product has some unusual shape requirements. For examples of plug-assisted blow molding descriptions, see U.S. Pat. No. 3,849,028 of Van der Greg et al. and Japanese Patent publication 56-164,817 of Sumitomo Bakelite.
A final series of patents describes the combined extrusion and biaxial orientation of thermoplastic tubing. See for example U.S. Pat. No. 3,182,355 of Arnaudin, Jr.
In order to produce a thermoplastic laboratory beaker or other open-ended container that will be used to contain fluids that will be heated from the bottom, it is imperative that the bottom of the beaker or container be thin and flat. Typically, fluid containing beakers are heated on a hot plate. To enhance the transfer of heat from the hot plate to the fluid within the container, the more beaker surface contacting the plate and the thinner the walls of the bottom of the container the more efficient the heat transfer. None of the thermoplastic beakers currently available combine all of the following characteristics desireable in such a product: 1) generally chemically inert; 2) heat stable up to 250.degree. C.; 3) flat and thin bottom; and 4) generally inexpensive to produce.
Another common problem with each of these processes is that the overall dimensions of the oriented articles is very difficult to control within exacting standards. In particular, it is extremely difficult to produce round objects with consistent diameters or non-round objects with consistent perimeter dimensions. For many purposes these variations in dimension are not significant. However, when utilizing such techniques to form seamless belts, for example (accomplished by slicing tubular sections of biaxially oriented material) close tolerances can be critical.